Set or clear multiselect picklist values from process builder

PBClearMultiPicklist.apxc

public class PBClearMultiPicklist {
    public class Params {
        @InvocableVariable(label='Object ID' required=true)
        public String sobjectId;
        
        @InvocableVariable(label='Picklist Field API Name' required=true)
        public String fieldName;
        
        @InvocableVariable(label='Value' required=true)
        public String value;
    }

    @InvocableMethod(
        label='Clear a multiselect picklist value'
        description='Removes a selected value from a multiselect picklist without changing other selected values.'
    )
    public static void clearPicklistValue(List<Params> ps){
        Params p = ps[0];
        Id sobjectId = (Id)(p.sobjectId);
        String fieldName = p.fieldName;
        String value = p.value;
        
        // Fetch object by ID
        Schema.SObjectType sobjectType = sobjectId.getSObjectType();
        String sobjectName = sobjectType.getDescribe().getName();
        SObject record = Database.query('Select Id,' + fieldName + ' From ' + sobjectName + ' Where Id = :sobjectId');
        
        // Get existing field values into set
        String prevStr = (String)record.get(fieldName);
        Set<String> values = new Set<String>(prevStr.split(';'));
        
        // Add new value
        values.remove(value);
        
        // Rebuild value string from set
        List<String> sorted = new List<String>(values);
        sorted.sort();
        String newStr = String.join(sorted, ';');
        record.put(fieldName, newStr);
        
        update record;
    }
}

PBSetMultiPicklist.apxc

public class PBSetMultiPicklist {
    public class Params {
        @InvocableVariable(label='Object ID' required=true)
        public String sobjectId;
        
        @InvocableVariable(label='Picklist Field API Name' required=true)
        public String fieldName;
        
        @InvocableVariable(label='Value' required=true)
        public String value;
    }

    @InvocableMethod(
        label='Set a multiselect picklist value'
        description='Adds a selected value to a multiselect picklist without changing other selected values.'
    )
    public static void setPicklistValue(List<Params> ps){
        Params p = ps[0];
        Id sobjectId = (Id)(p.sobjectId);
        String fieldName = p.fieldName;
        String value = p.value;
        
        // Fetch object by ID
        Schema.SObjectType sobjectType = sobjectId.getSObjectType();
        String sobjectName = sobjectType.getDescribe().getName();
        SObject record = Database.query('Select Id,' + fieldName + ' From ' + sobjectName + ' Where Id = :sobjectId');
        
        // Get existing field values into set
        String prevStr = (String)record.get(fieldName);
        Set<String> values = new Set<String>(prevStr.split(';'));
        
        // Add new value
        values.add(value);
        
        // Rebuild value string from set
        List<String> sorted = new List<String>(values);
        sorted.sort();
        String newStr = String.join(sorted, ';');
        record.put(fieldName, newStr);
        
        update record;
    }
}

Here is an example test class for this apex class.
http://salesforce.stackexchange.com/questions/138902/method-does-not-exist-or-incorrect-signature-calling-public-class-from-test-cl